The paths
to extinction for a species are many, and they are traveled in many ways.
Some routes, both natural and man-caused, are obvious. Among natural causes
that could lead to extinction for a species are the appearance of a new
disease that eliminates individuals more rapidly than the host species
can develop a resistance. Or natural climate change might occur at rates
that certain species cannot adjust to. Or major natural disasters, such
as volcanoes or tsunamis, may cause the extinction of species restricted
to oceanic islands.

Of the many
ways that humans have been responsible for the extinction of plant and
animal species, such as unconstrained removal from the wild or the introduction
of diseases, parasites, or predators, one assault on the environment stands
above all others--habitat destruction. The loss of natural habitat is
a death knell for many native species. The fragmentation of natural habitats
into smaller and smaller parcels as a result of commercial urbanization
and agriculture throughout much of the country and the world will gradually
spell the end for thousands if not millions of species.

A study
in Finland provided documentation of a far less obvious phenomenon that
can also result in extinction of natural populations--inbreeding. Research
by Ilik Saccheri and colleagues at the University of Helsinki implicated
inbreeding as a contributor to declines and ultimate extinction of small,
isolated populations of a butterfly called the Glanville fritillary. Inbreeding,
in which close relatives in a population mate and produce offspring, has
long been considered a less-than-optimal situation because of the greater
likelihood that deleterious genes will be expressed.

The study
in Finland was able to examine the total effects inbreeding can have when
an entire population inbreeds. In a region having more than 1,600 meadows
suitable for small populations of the Glanville fritillary butterfly,
the investigators searched meadows for larvae of the butterflies. The
presence of larvae, of course, indicated that adults were breeding and
producing offspring.

During a
four-year study period, the researchers located more than 500 meadows
with larvae, then selected 42 populations for genetic tests to determine
how variable each individual and population was genetically. Although
sophisticated laboratory techniques must be used to assess the genetic
makeup of individuals, once the analysis is accomplished, comparing the
genetic variability present is simple. An assumption is that when there
is low genetic variability among offspring, the parents were closely related.
In contrast, high variability in genetic makeup among offspring is considered
to be a healthy sign within a population. With this in mind, the scientists
compared the genetic makeup of butterfly larvae, the offspring, in the
42 different meadows and followed the fate of each population to see if
it persisted in the meadow or became extinct.

The researchers
found that the lower the genetic variability of a population, the higher
the probability of its extinction. That is, the most-inbred butterfly
populations were most likely to decline in numbers and eventually go extinct.
Several components in the life cycle of the butterflies were identified
as being adversely affected by inbreeding. For example, the survival of
larvae was lower. Likewise, those reaching adulthood did not live as long.
Also, the hatching rate of the eggs was lower in the populations with
high inbreeding.

The severe
consequences of inbreeding demonstrated in the Glanville fritillary butterflies
in Finland could occur naturally. But such situations are anything but
natural when human beings isolate animal species into small populations
and create greater opportunities for inbreeding. By fragmenting habitats
so that normally widespread populations are separated into small populations,
we create situations in which inbreeding is more likely to occur. We now
know that such inbreeding can lead to extinction.

Habitat
fragmentation occurs every day throughout the United States and the world.
When modification of a forest results in two or more small wooded areas
instead of one large area, animal species may be isolated into small inbreeding
populations. The consequences of such isolation can adversely affect the
long-term welfare of some species. Knowing this, we have one more reason
for curtailing the fragmentation of habitats.